bootmgr compressed...which app to use from UBCD, and how?

Hi,

I have my son's class PC at home, - fixed a soundcard driver problem, then made the schoolboy error of compressing the entire HDD to create disc space.

Of course, that included the boot files.

On startup, I get: bootmgr compressed (& instructions to try reboot)

With UBCD inserted I can navigate the main menu, but not much idea which app could help, nor of the exact steps needed.

It's running Windows 7 Home Premium, and I don't have the disc (could possibly get it from school, but not sure).

I would be massively grateful if anyone could help.

Thanks,

Jon